Switching power supply device and switching method

ABSTRACT

A current flowing through a reactor flows through a resistor, which generates a voltage in accordance with the value of the current. As the voltage generated by the resistor is greater than or equal to the threshold of a transistor, the transistor is in an on state. As the current flowing through the reactor decreases and the voltage generated by the resistor becomes lower than the threshold of the transistor, the transistor turns off and an NMOS turns on. Accordingly, the gate voltage of an NMOS is decreased by a diode, ensuring that the NMOS is turned off before the current flowing through the secondary winding of a transformer becomes zero.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching power supply device and aswitching method used by the same.

2. Description of the Related Art

A conventional switching power supply (switched-mode power supply)device is disclosed in Unexamined Japanese Patent Application KOKAIPublication No. 2004-135415.

FIG. 13 shows a circuit diagram illustrating a conventional switchingpower supply device given in the Publication.

The switching power supply device comprises a main switching element Q1,a synchronous rectification switching element Q2, a series circuit 26,and a synchronous rectification switching control circuit 27.

The main switching element Q1 switches (turns on and off) a currentwhich flows through the primary winding LP, of a transformer T1. Thesynchronous rectification switching element Q2 is connected between thesecondary winding, LS, of the transformer T and a load. The seriescircuit 26 includes a synchronous-rectification-inductance element L1and a diode D1, and is connected to the secondary winding LS of thetransformer T1 in parallel. The synchronous rectification switchingcontrol circuit 27 includes a diode D2, a capacitor C51, and atransistor Q5.

The synchronous rectification switching element Q2 turns off in an onperiod of the main switching element Q1, and stores or accumulateselectrical power in the transformer T1 and thesynchronous-rectification-inductance element L1. In tern, thesynchronous rectification switching element Q2 turns on in an off periodof the main switching element Q1, and permits the stored electricalpower to be released. Before the release of the electrical power storedin the transformer T1 is completed, the action of the diode D1 causesthe synchronous-rectification-inductance element L1 to complete therelease of the stored electrical power. In accordance with a voltage ata node A between the synchronous-rectification-inductance element L1 andthe diode D1, the diode D2 in a synchronous rectification switchingcontrol circuit 27 detects that the stored electrical power of thesynchronous-rectification-inductance element L1 has been released, andturns off the synchronous rectification switching element Q2.

In the switching power supply device, even if the release of the storedelectric power of the transformer T1 is completed, the voltage at thenode A may not drop instantaneously. To be more precise, the voltage atthe node A may not drop instantaneously after the release of the storedelectric power from the transformer T1 is completed because of theeffect of the inductance of the synchronous-rectification-inductanceelement L1 and the capacity of the synchronous-rectification controlcircuit 27, or the parasitic capacity of the synchronous rectificationinductance element L1. The delay of the reduction in voltage at the nodeA may keep the synchronous rectification switching element Q2 turned onafter the release of the stored electrical power of the transformer T1is completed. This may deteriorate the efficiency, and damage theelement.

SUMMARY OF THE INVENTION

Accordingly, the objects of the present invention to realize a switchingpower supply device in which the rectifying switch turns on and off atadequate timings.

To achieve the object, a switching power supply device of the inventioncomprises:

a transformer with a primary and a secondary winding;

a main switching element which switches a current flowing through theprimary winding;

a controller which controls an operation of the main switching element;

a smoothing circuit;

a rectifying switching element which connects and disconnects betweenthe secondary winding and the smoothing circuit; and

a rectifying-element drive circuit which drives the rectifying switchingelement, and includes

a reactor which is connected to the secondary winding in parallel,stores energy during an on period of the main switching element, andreleases the stored energy during an off period of the main switchingelement, and

a drive circuit which detects a current value of a current flowingthrough said reactor and turns on the rectifying switching element inthe off period of the main switching element when the current flowingthrough the reactor is greater than or equal to a predetermined value,and turns off the rectifying switching element when the current flowingthrough the reactor is less than the predetermined value.

By employing such a structure, the rectifying switching element turns onby the drive circuit when the current which flows through the reactor,connected to the secondary winding of the transformer in parallel, isgreater than or equal to the predetermined value. The rectifyingswitching element turns off when the current which flows through thereactor becomes smaller than the predetermined value. This makes itpossible to prevent making the on period in which the rectifyingswitching element is on longer needlessly.

The driving circuit may include a current detection circuit whichdetects the current value of the current flowing through the reactor.The current detection circuit may include a current detection resistorwith one end connected to the reactor and an other end connected to thesecondary winding, and a transistor with a control electrode, and afirst and a second conduction electrode which change conduction statesbased on a signal supplied to the control electrode, the controlelectrode (base) being connected to the reactor, the first conductionelectrode (emitter) being connected to the secondary winding, and

the drive circuit may turn off the rectifying switching element, basedon a voltage at said second conduction electrode of said transistor whena voltage drop at the current detection resistor due to the currentflowing through the reactor becomes lower than a threshold of thetransistor.

The transistor may be a bipolar transistor whose base, emitter, andcollector respectively correspond to the control electrode, the firstconduction electrode, and the second conduction electrode. Thetransistor may be a MOS transistor whose gate, source, and drainrespectively correspond to the control electrode, the first conductionelectrode, and the second conduction electrode.

The drive circuit may include an off-control switch which has a mainterminal with one end (source) connected to the secondary winding, andan other end (drain) connected to a control terminal of the rectifyingswitching element, and a control terminal (gate) connected to anauxiliary winding, connected to the secondary winding in series, and thesecond conduction electrode (collector) of the transistor, turn on,reducing a voltage at the control terminal of said rectifying switchingelement, when the transistor is turned off.

The drive circuit may include a current bypass diode whose anode andcathode respectively connected to the reactor, and the second conductionelectrode of the transistor.

The drive circuit may include a hysteresis circuit which includes aresistor and a diode, is connected between the reactor and the other endof the current path of the off-control switch, and permits the currentflowing through the reactor to partly flow through the off-controlswitch during an on period of the off-control switch.

The drive circuit may include a bias circuit which ensures a currentflow to the current detection circuit from the auxiliary winding or thecontrol terminal of the rectifying switching element via a resistor.

The rectifying-element drive circuit may include a capacitor which isconnected between the control terminal of the rectifying switchingelement and the auxiliary winding, and has a voltage-reduction functionof reducing a voltage at the control terminal of the rectifyingswitching element, or has functions of a drive function of driving therectifying switching element and the voltage-reduction function.

The drive circuit may include a drive transistor whose emitter, base,and collector are respectively connected to the control terminal of therectifying switching element, an other end of a main terminal of theoff-control switch, and the auxiliary winding, and a resistor and aZener diode are connected between the base and the collector of saiddrive transistor.

The drive circuit may include a current detection circuit which detectsa current value of the current flowing through the reactor and a gatecircuit such as an NOR circuit which turns on and off the rectifyingswitching element based on an output signal of the detection result ofsaid current detection circuit when said main switching element is off.

The current detection circuit may includes

a current detection resistor having one end connected to the reactor andan other end connected to the secondary winding, and

a comparator which compares a voltage generated by the current detectionresistor with a predetermined voltage, and

the drive circuit may turn on and off the rectifying switching elementbased on an output signal of the comparator when said main switchingelement is off.

To achieve the object, a switching method according to the inventioncomprises the steps of:

intermittently supplying a current to a primary winding of atransformer; and

turning on a rectifying switching element to supply an output of asecondary winding of the transformer to a smoothing circuit via arectifying switching element by in a period in which no current flowsthrough the first winding and when a current value of a current flowingthrough a reactor, connected to the secondary winding of the transformerin parallel, is greater than or equal to a predetermined value, based onthe current flowing through the reactor, and turning off the rectifyingswitching element when the current value of the current flowing throughthe reactor is less than the predetermined value.

According to the invention, when the current which flows through thereactor is less than the predetermined value, the rectifying switchingelement turns off. Therefore, making the time, during which therectifying switching element is on, longer needlessly can be prevented,and the efficiency is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

These objects and other objects and advantages of the present inventionwill become more apparent upon reading of the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a circuit diagram illustrating a switching power supply deviceaccording to a first embodiment of the invention;

FIGS. 2A to 2H are waveform charts for explaining an operation of theswitching power supply device;

FIG. 3 is a circuit diagram illustrating a switching power supply deviceaccording to a second embodiment of the invention;

FIG. 4 is a circuit diagram illustrating a switching power supply deviceaccording to a third embodiment of the invention;

FIG. 5 is a circuit diagram illustrating a switching power supply deviceaccording to a fourth embodiment of the invention;

FIG. 6 is a circuit diagram illustrating a switching power supply deviceaccording to a fifth embodiment of the invention;

FIG. 7 is a circuit diagram illustrating a switching power supply deviceaccording to a sixth embodiment of the invention;

FIG. 8 is a circuit diagram illustrating a switching power supply deviceaccording to a seventh embodiment of the invention;

FIG. 9 is a circuit diagram illustrating a switching power supply deviceaccording to an eighth embodiment of the invention;

FIG. 10 is a circuit diagram illustrating a switching power supplydevice according to a ninth embodiment of the invention;

FIG. 11 is a circuit diagram illustrating a switching power supplydevice according to a tenth embodiment of the invention;

FIG. 12 is a circuit diagram illustrating a switching power supplydevice according to an eleventh embodiment of the invention; and

FIG. 13 is a circuit diagram illustrating a conventional switching powersupply apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

FIG. 1 is a circuit diagram illustrating a switching (switched-mode)power supply device according to a first embodiment of the presentinvention.

The switching power supply device is a flyback converter which includesa transformer 2 connected to a direct-current (DC) power source 1.

The hot side of a primary winding 2 a of the transformer 2 is connectedto the anode of the DC power source 1. The cold side of the primarywinding 2 a is connected to the drain of an N-channel type MOS (MetalOxide Semiconductor) transistor (hereinafter, “NMOS”) 3 which is a mainswitching element. A controller 4 is connected to the gate of the NMOS3, and supplies a control signal to that gate. The source of the NMOS 3is connected to the cathode of the DC power source 1.

The transformer 2 further includes a secondary winding 2 b and anauxiliary winding 2 c which are electromagnetically coupled to theprimary winding 2 a via a core. The auxiliary winding 2 c is connectedto the cold side of the secondary winding 2 b in series.

The hot side of the secondary winding 2 b is connected to one end of areactor 5, the negative electrode of a smoothing capacitor 6, and theground GND. The cold side of the secondary winding 2 b is connected tothe source of an NMOS 7 which serves as a synchronous-rectificationswitching element. The drain of the NMOS 7 is connected to the positiveelectrode of the smoothing capacitor 6. An output terminal Tout isconnected to the positive electrode of the smoothing capacitor 6. Anoutput voltage Vo is supplied to a non-illustrated load from the outputterminal Tout.

The other end of the reactor 5 is connected to the anode of a backflowprevention diode 8. The cathode of the diode 8 is connected to one endof a current-detection resistor 9. The other end of thecurrent-detection resistor 9 is connected to the cold side of thesecondary winding 2 b. Accordingly, a series circuit of the reactor 5,the diode 8, and the resistor 9 is connected to the secondary winding 2b in parallel.

The cold side of the auxiliary winding 2 c connected to the secondarywinding 2 b is connected to the anode of a diode 10. The cathode of thediode 10 is connected to one ends of resistors 11 and 12, and thecollector of an NPN transistor 13.

The other end of the resistor 12 is connected to the base of thetransistor 13. The emitter of the transistor 13 is connected to theanode of a diode 14 and one end of a resistor 15. The cathode of thediode 14 is connected to the base of the transistor 13. The other end ofthe resistor 15 is connected to the gate of the NMOS 7.

The base of the transistor 13 is further connected to the drain of anNMOS 16. The source of the NMOS 16 is connected to the cold side of thesecondary winding 2 b.

The other end of the resistor 11 is connected to the anode of a diode17. The cathode of the diode 17 is connected to the gate of the NMOS 16,and the collector of the NPN transistor 18. The base of the transistor18 is connected to a node between the diode 8 and the resistor 9. Theemitter of the transistor 18 is connected to the cold side of thesecondary winding 2 b. An NPN bipolar transistor may be used instead ofthe NMOS 16. In this case, the collector, base, and emitter of the NPNtransistor substituting the NMOS 16 are respectively connected to thebase of the NPN transistor 13, the cathode of the diode 17, and the coldside of the secondary winding 2 b.

Next, an operation of the switching power supply device shown in FIG. 1will be explained.

FIGS. 2A to 2H are waveform charts for explaining the operation of theswitching power supply device.

The NMOS 3 turns on and off in response to a control signal suppliedfrom the controller 4. In a period in which the NMOS 3 is in the onstate, that is, when a drain-source voltage Vds of the NMOS 3illustrated in FIG. 2A is (almost) zero volt, a primary current Id flowsthrough (across) the primary winding 2 a of the transformer 2 asillustrated in FIG. 2B.

Given that the length of the on period during which the NMOS 3 is on isTon, the inductance of the primary winding 2 a is Lp, and the outputvoltage of the DC power source 1 is Vin, the transformer 2 stores anenergy of (Vin²/2Lp)Ton in the on period of the NMOS 3.

As illustrated in FIG. 2C, the secondary winding 2 b generates a voltageVT from its hot side in the on period of the NMOS 3, and the voltage onthe hot side becomes higher than the voltage on the cold side. Theauxiliary winding 2 c generates a voltage from its hot side, and thevoltage on the hot side becomes higher than the voltage on the coldside. As the voltage at the hot side of the auxiliary winding 2 cbecomes higher than that on the cold side of the auxiliary winding 2 c,the transistor 13 is set in an off state. Accordingly, as illustrated inFIG. 2H, the gate-source voltage Vgs of the NMOS 7 is not generated, andthe NMOS 7 is set in an off state.

Given that the number of turns on the primary winding 2 a of thetransformer 2 is np, and the number of turns on the secondary winding 2b is ns, a voltage VT generated at the secondary winding 2 b in the onperiod of the NMOS 3 can be expressed by an equation:VT=(ns/np)Vin.

As the voltage on the hot side of the secondary winding 2 b becomeshigher than that on the cold side, a current IL flows to the diode 8 andthe resistor 9 from the reactor 5 as illustrated in FIG. 2E. The currentIL increases in the on period of the NMOS 3.

When the voltage drop across the resistor 9 due to the flow of thecurrent IL through the resistor 9 becomes larger than the threshold ofthe transistor 18, the transistor 18 comes to an on state. Accordingly,the voltage drop across the resistor 9 changes as illustrated in FIG.2F. With the sum of the forward voltage of the diode 8 and the voltagedrop VR across the resistor 9 or a base-emitter voltage VR of thetransistor 18 being ΔV(t), a voltage of VT−ΔV(t) is applied to thereactor 5.

When the NMOS 3 turns off based on the control signal of the controller4, the secondary winding 2 b and auxiliary winding 2 c of thetransformer 2 generate voltages higher than the voltages on the hotsides from the cold sides. Because of the voltage at the secondarywinding 2 b of the transformer 2, the capacitor 6 is charged through aparasitic diode of the NMOS 7.

Immediately after the NMOS 3 has turned off, the transistor 18 is in anon state and the NMOS 16 is in an off state, so that the voltage on thecold side of the auxiliary winding 2 c becomes higher than that on thehot side of the auxiliary winding 2 c. This increases the base voltageof the transistor 13 through the resistor 12, turning on the transistor13.

The on action of the transistor 13 causes the NMOS 7 to turn on. As theNMOS 7 turns on, the energy stored in the transformer 2 is released as asecondary current IT through the NMOS 7 as illustrated in FIG. 2D. Thecapacitor 6 is charged by the secondary current IT.

The secondary current IT decreases with the time. The inclination of thedecrease in secondary current IT may be expressed as (Vo²/2LS)t². Here,LS denotes the inductance of the secondary winding 2 b.

The numbers of turns, np and ns of the primary and secondary windings 2a and 2 b, the inductance LP of the primary winding 2 a and theinductance LS of the secondary winding 2 b have a relationship expressedby an equation:LS=(ns ² /np ²)LP.

Accordingly, a time t until the secondary current IT stops flowing canbe expressed by an equation:t=(nsVin/npVo)Ton.

The reactor 5 releases the energy stored in the on period of the NMOS 3via the diode 8 when the NMOS 3 turns off. Provided that the sum of theforward voltage of the diode 8 and the voltage drop at the resistor 9 orthe base-emitter voltage of the transistor 18 is ΔV(t)on, and theinductance of the reactor 5 is L and the on period of the NMOS3 is Ton,the current IL which flows through the reactor 5 at the end of the onperiod of the NMOS 3 can be expressed by an equation:IL=(VT−ΔV(t)on)Ton/L.

The current IL which flows through the reactor 5 decreases in an offperiod in which the NMOS 3 is off.

Provided that the sum of the forward voltage of the diode 8 and thevoltage drop across the resistor 9 or the base-emitter voltage of thetransistor 18 is ΔV(t)off, a time during which the current IL flowingthrough the reactor 5 becomes zero will now be expressed by an equation:

$\begin{matrix}\begin{matrix}{t = {\left( {{VT} - {\Delta\;{V(t)}\mspace{14mu}{on}}} \right)\mspace{14mu}{{Ton}/\left( {{Vo} + {\Delta\;{V(t)}\mspace{14mu}{off}}} \right)}}} \\{= {\left( {{\left( {{ns}/{np}} \right){Vin}} - {\Delta\;{V(t)}\mspace{14mu}{on}}} \right)\mspace{14mu}{{Ton}/{\left( {{Vo} + {\Delta\;{V(t)}\mspace{14mu}{off}}} \right).}}}}\end{matrix} & (1)\end{matrix}$

ΔV(t)on and ΔV(t)off are sufficiently smaller values than a voltage V₂generated by the secondary winding 2 b and the voltage Vo. Accordingly,as illustrated in FIGS. 2D and 2E, the current IL which flows throughthe reactor 5 becomes zero slightly faster than the secondary currentIT.

As the current IL flowing through the reactor 5 decreases and thevoltage drop VR across the resistor 9 becomes lower than the thresholdof the transistor 18, the transistor 18 turns off. Accordingly, the gateof the NMOS 16 is enabled (applied with a high voltage) via the resistor11 and the diode 17, causing the NMOS 16 to turn on. The on action ofthe NMOS 16 sets the transistor 13 in an off state, so that electricalcharges are drawn from the gate of the NMOS 7 via the diode 14. Thiscauses the gate-source voltage Vgs of the NMOS 7 to drop, thus turningoff the NMOS 7.

The current IL which flows through the reactor 5 and determines a timingat which the NMOS 7 turns off can be set by an equation:IL=V _(BE) /R ₉

where V_(BE) is the base-emitter voltage of the transistor 18, and R₉ isthe value of resistance of the resistor 9.

Increasing the value of resistance R₉ makes it possible to set thetiming at which the NMOS 7 turns off immediately before the current ILthrough the reactor 5 becomes zero. Hence, because of the relationshipexpressed by the equation (1), the NMOS 7 turns off before the secondarycurrent IT becomes zero. After the NMOS 7 turns off, the parasitic diodeof the NMOS 7 performs rectification. Since the secondary current IT isbasically a triangular wave, even if the current IL is rectified by theparasitic diode, the product of the current and the time during thatperiod is just a few percentages of the total current, which does notaffect the loss substantially.

The voltage across the reactor 5 which is connected to the secondarywinding 2 b as in the manner illustrated in FIG. 1 is (Vo+ΔV) in aperiod during which the energy in the reactor 5 is released, and becomeszero after the release of the energy is finished. Detecting the voltageacross the reactor 5 makes it possible to turn off the NMOS 7immediately before the secondary current IT becomes zero. The voltageacross the reactor 5, however, does not instantaneously drop because ofthe inductance of the reactor 5 and the capacity of a voltage detectioncircuit, or the parasitic capacity of the reactor 5. Because of thisdelay, there is a negative possibility that the NMOS 7 keeps turned onafter the secondary current IT becomes zero.

Designing the power supply device in view of such a delay requires thatthe value of resistance of the resistor 9 which is connected to thereactor 5 in series be increased, and a plurality of diodes 8 beconnected in series. Because the voltage drop is changed by the load andtemperature, the synchronous rectification period should be so designedshorter. Reducing the inductance L of the reactor 5 makes the loweringspeed of the voltage across the reactor 5 faster, but the current ILincreases accordingly, thereby increasing the loss.

When the NMOS 7 is turned off based on a change in voltage across thereactor 5, therefore, the loss may be increased and the packaging spacemay be increased, making it difficult to achieve cost reduction. Incontrast, the switching power supply device of the embodiment detectsthe value of the current IL which flows through the reactor 5 by meansof the resistor 9, and turns off the NMOS 7 based on that current value.Accordingly, it is possible to surely turn off the NMOS 7 before thesecondary current IT becomes zero without being affected by the delaycaused by the inductance of the reactor 5. This makes it possible torealize an efficient switching power supply device at a low cost.

(Second Embodiment)

FIG. 3 is a circuit diagram illustrating a switching power supply deviceaccording to a second embodiment of the invention, and components commonto those of the first embodiment in FIG. 1 will be denoted by the samereference numerals.

The switching power supply device includes a current-bypass diode 20together with the structure shown in FIG. 1. The remaining structuresare the same as those of the switching power supply device of the firstembodiment.

The anode of the diode 20 is connected to the node between the one endof the reactor 5 and the anode of the diode 8. The cathode of the diode20 is connected to the collector of the transistor 18.

A basic operation of the switching power supply device is the same asthat of the first embodiment. The current IL which flows through thereactor 5 after the transistor 18 turns on, however, flows into thecollector of the transistor 18 as well as the base thereof because thediode 20 is provided between the one end of the reactor 5 and thecollector of the transistor 18.

In the switching power supply device of the first embodiment, thecurrent IL which flows through the reactor 5 entirely becomes the basecurrent of the transistor 18. In general, the absolute maximum rating ofa base current of a transistor is smaller than its collector current,and a control transistor with a small signal cannot increase the currentIL which flows through the reactor 5.

It is possible to limit the value of the current IL which flows throughthe reactor 5 within the standard by increasing the inductance L of thereactor 5. Too much limitation, however, may not obtain a sufficientcurrent gain at the transistor 18. Accordingly, it is not preferablethat all of the current IL of the reactor 5 should flow into the base ofthe transistor 18.

In the switching power supply device of the embodiment, the current ILwhich flows through the reactor 5 partly flows into the collector of thetransistor 18. This makes it possible to prevent the base current of thetransistor 18 from exceeding the absolute maximum rating. In this case,the transistor 18 performs class A operation in such a way that thecollector-emitter voltage of the transistor 18 becomes equal to thebase-emitter voltage thereof. Accordingly, in a case where the NMOS 16has a low threshold, or a bipolar transistor is used instead of the NMOS16, it is necessary to divide the collector voltage of the transistor 18by using a voltage-dividing resistor or the like, and apply the dividedvoltage to the gate of the NMOS 16 or the base of the bipolartransistor.

(Third Embodiment)

FIG. 4 is a circuit diagram illustrating a switching power supply deviceaccording to a third embodiment of the invention, and components commonto those of the second embodiment in FIG. 3 will be denoted by the samereference numerals.

The switching power supply apparatus replaces the transistor 18 of thesecond embodiment by a NMOS 21. The remaining structures are the same asthose of the switching power supply device of the second embodiment.

The gate of the NMOS 21 is connected to the node between the cathode ofthe diode 8 and the resistor 9, and the drain of the NMOS 21 isconnected to the cathodes of the diodes 17, 20, and the gate of the NMOS16. The source of the NMOS 21 is connected to the cold side of thesecondary winding 2 b of the transformer 2.

In the switching power supply device, in a case where the voltage dropat the resistor 9 due to the current IL flowing from the reactor 5 ishigher than the threshold of the NMOS 21, the NMOS 21 turn on. In a casewhere the voltage drop at the resistor 9 due to the current IL becomeslower than the threshold of the NMOS 21, the NMOS 21 turns off. When theNMOS 21 turns off, the NMOS 16 turns on. Accordingly, the NMOS 7 turnsoff, and the synchronous rectification is stopped.

The gate voltage of the NMOS 21 differs from the base-emitter voltage ofthe transistor 18, and is not to be clamped to a constant voltage evenif the NMOS 21 is in an on state. Accordingly, by the equation (1),increments of the current IL flowing from the reactor 5 and the voltagedrop at the resistor 9 significantly shortens a time until the currentIL flowing from the reactor 5 becomes zero in comparison with thesecondary current IT flowing through the secondary winding 2 b. As thediode 20 flows the current IL from the reactor 5 into the drain of theNMOS 21 after the NMOS 21 turns on, the diode 20 so functions as tosuppress the voltage drop at the current-detection resistor 9 within 1to 2 V or so in the vicinity of the threshold of the NMOS 21. Therefore,greatly shortening the time until the current IL becomes zero can beprevented in comparison with the secondary current IT. In this case too,when the NMOS 16 has a low threshold, or a bipolar transistor is usedinstead of the NMOS 16, it is necessary to divide the drain voltage ofthe NMOS 21 by using a voltage-dividing resistor or the like, and applythe divided voltage to the gate of the NMOS 16 or the base of thebipolar transistor.

(Fourth Embodiment)

FIG. 5 is a circuit diagram illustrating a switching power supply deviceaccording to a fourth embodiment of the invention, and components commonto those of the second embodiment in FIG. 3 will be denoted by the samereference numerals.

The switching power supply device includes a diode 23 and a resistor 24.The remaining structures are the same as those of the switching powersupply device of the second embodiment.

The anode of the diode 23 is connected to the one end of the reactor 5,the anodes of the diode 8, 20. The cathode of the diode 23 is connectedto one end of the resistor 24, and the other end of the resistor 24 isconnected to node between the base of the transistor 13 and the drain ofthe NMOS 16.

In flyback converters, ringing is generated when a main switchingelement is in an off state and the release of energies in thetransformer 2 is finished. In the switching power supply apparatus ofthe second embodiment, ringing is generated when the NMOS 3 is in an offstate and the release of the energy in the transformer 2 is finished,and a sine-wave voltage which is equal to the output voltage Vo isgenerated at the secondary winding 2 b. The reactor 5 also stores andreleases the energy by that sine-wave voltage.

To do synchronous rectification maximally, setting the resistance valueof the resistor 9 in such a way that the NMOS 7 keeps being in an onstate just before the current IL flowing through the reactor 5 becomeszero may cause the transistor 18 to be maintained in the on state in theperiod of the ringing, and the NMOS 7 may be so driven as to turn on.

The switching power supply device of the embodiment of the invention canresolve the problem of the switching power supply device as such aflyback converter.

When the release of the energy in the transformer 2 is finished, and thesecondary current IT flowing from the secondary winding 2 b becomeszero, the current IL flowing from the reactor 5 becomes also zero. Thiscauses the transistor 18 to turn off, and the NMOS 16 to turn on.Subsequently, because of the generation of the ringing, in flowing thecurrent IL again through the reactor 5 by the ringing voltage, thecurrent IL flows into the NMOS 16 via the diode 23 and the resistor 24in addition to the resistor 9.

Given that the on resistance of the NMOS 16 is, for example, 200 mΩ, andthe current maximally flowing through the NMOS 16 is 50 mA, thedrain-source voltage of the NMOS 16 is 10 mV, and is significantlysmaller than approximately 0.6V threshold between the base and emitterof the transistor 18. Consequently, with the drain-source voltage of theNMOS 16 being neglected, and the forward voltage of the diode 8 beingequal to that of the diode 23, the transistor 18 is to be turn on whenthe voltage drop due to the combined resistance of the resistor 9 andthe resistor 24 becomes lower than the threshold of the transistor 18.In fact, with the values of resistances of the resistor 9 and theresistor 24 being R₉ and R₂₄, respectively, and when the current ILbecomes likeIL2=VBE(R ₉ +R ₂₄)/(R ₉ ·R ₂₄)

the transistor 18 is to turn on.

Because of the capacitance between the gate and source of the NMOS 16,the gate voltage of the NMOS 16 is held until the transistor 18 turnson, and setting the value of resistance R₂₄ of the resistor 24 in such away that IL2 becomes larger than the current flowing through the reactor5 due to the ringing prevents the NMOS 7 from being driven so as to turnon in the ringing period.

In a case where a bipolar transistor is used instead of the NMOS 16, thesimilar effects can be obtained by connecting a capacitor between thecathode of the diode 17 and the cold side of the secondary winding 2 band maintaining the base-voltage of the bipolar transistor by thecapacitor, thereby keeping flowing the base current.

As the NMOS 3 as the main switching element turns on, the voltage at thehot side terminal of the secondary winding 2C becomes higher than thatof the cold side terminal of the secondary winding 2C. Thus, there is novoltage to drive the gate of the NMOS 16 through the diode 10, theresistor 11, and the diode 17, and the NMOS 16 turns off, and no currentflows through the resistor 24. Accordingly, it is possible to set thecurrent IL which flows through the reactor 5 when the transistor 18 isin an off state by the value of resistance of the resistor 9, and turnthe NMOS 7 off just before the current IL flowing through the reactor 5becomes zero.

In a case of a flyback converter which feeds back the state of the loadand adjusts the length of the on period of the main switching element,when the load is light, the secondary current IT of the secondarywinding 2 b may decrease. In this case, the loss may decrease. However,the electrical power for driving the NMOS 7 is the same in both of theheavy load and light load. So sometimes, the loss because of doingsynchronous rectification may become larger than that of not-doingsynchronous rectification.

In the switching power supply device of the embodiment, when the onperiod of the NMOS 3 with a light load is short, the current IL flowingthrough the reactor 5 decreases, and the transistor 18 does not turn on,thus undoing the synchronous rectification. Therefore, the switchingpower supply device of the embodiment can achieve an effect such thatthe loss with a light load is reduced.

(Fifth Embodiment)

FIG. 6 is a circuit diagram illustrating a switching power supply deviceaccording to a fifth embodiment of the invention, and components commonto those of the second embodiment in FIG. 3 will be denoted by the samereference numerals.

The switching power supply device comprises a diode 25, resistors 26,27, a diode 28, and a capacitor 29, and the structures which are thesame as those of the switching power supply device of the secondembodiment.

The anode of the diode 25 is connected to the node between the one endof the resistor 15 and the emitter of the transistor 13, and the cathodeof the diode 25 is connected to one end of the resistor 26. The otherend of the resistor 26 is connected to one end of the resistor 27, theanode of the diode 28, and one electrode of the capacitor 29. The otherend of the resistor 27 is connected to the base of the transistor 18.The cathode of the diode 28 is connected to the collector of thetransistor 18. The other electrode of the capacitor 29 is connected tothe cold side of the secondary winding 2 b of the transformer 2.

The switching power supply device achieves the similar effects to thoseof the switching power supply device of the fourth embodiment, andprevents the NMOS 7 to turn on when ringing is generated and the load isnot heavy.

When the NMOS 3 which is the main switching element is in an on state,or no voltage is applied to the gate of the synchronous rectificationNMOS 7 in a ringing period, only the current IL which flows through thereactor 5 flows into the resistor 9. With the value IL3 of the currentIL which flows through the reactor 5 when the transistor 18 turns on inthat condition being expressed by the base-emitter voltage V_(BE) of thetransistor 18 and the value of resistance R₉ of the resistor 9, the IL3can be expressed as follows:IL3=V _(BE) /R ₉

Accordingly, by setting the value of resistance R₉ of the resistor 9 insuch a way that the current value IL3 becomes greater than the currentIL which flows through the reactor 5 in the ringing period and when theload is not heavy, it is possible to prevent the transistor 18 to turnon, thus preventing the NMOS 7 to turn on in the ringing period and whenthe load is not heavy.

In contrast, in a condition where the gate of the NMOS 7 is driven andthe NMOS 7 is in an on state, a bias current flows into the resistor 9from the gate of the NMOS 7 via the diode 25, and the resistors 26, 27.

At this time, because of the diode 28, the voltage at the node betweenthe resistor 26 and the resistor 27 becomes the sum of thecollector-emitter voltage V_(CE) of the transistor 18 and a forwardvoltage V_(F) of the diode 28. As mentioned above, at the transistor 18,as the collector-emitter voltage V_(CE) becomes equal to thebase-emitter voltage V_(BE), the node between the resistor 26 and theresistor 27 is clamped by the value of V_(BE)+V_(F). Therefore, when thetransistor 18 is in an on state, with the value of resistance of theresistor 27 being R₂₇, the bias current of V_(F)/R₂₇ flows through theresistor 9.

In a case where a voltage generated at the resistor 9 becomes lower thanthe threshold of the transistor 18 by the sum of the current IL whichflows through the reactor 5 and a current which is biased through theresistor 27, the transistor 18 turns off. Given that the value of thecurrent IL which flows through the reactor 5 when the transistor 18turns off being IL4, by setting the switching power supply device so asto satisfy the following equation:IL4+V _(F) /R ₂₇ =V _(BE) /R ₉that is, R ₂₇ =V _(F)(IL3−IL4),

it is possible to suppress that the NMOS 7 turns on when ringing isgenerated and the load is not heavy.

There is fear that the voltage at the node of the resistors 26, 27increases in a short period until the NMOS 16 turns on and the gatevoltage of the NMOS 7 lowers after the transistor 18 turns off, and thebias at the resistor 9 increases, thereby turning on the transistor 18again, but as the capacitor 29 delays increase of the voltage at thenode of the resistors 26, 27, and this makes it possible to prevent thetransistor 18 to turn on again.

It is possible to directly flow the bias current into the resistor 9from the gate of the NMOS 7 or the auxiliary winding 2 c by using aresistor and a diode without stabilizing the bias current which is to beflown into the resistor 9, in this case, however, it is necessary to setthe bias in view of the change in the voltage generated at the auxiliarywinding 2 c and the temperature characteristic of the transistor.

(Sixth Embodiment)

FIG. 7 is a circuit diagram illustrating a switching power supply deviceaccording to a sixth embodiment of the invention, and components commonto those of the fourth embodiment in FIG. 5 will be denoted by the samereference numerals.

This switching power supply device comprises a diode 30, a capacitor 31,a resistor 32, and the structures which are the same as those of theswitching power supply device of the fifth embodiment.

The anode of the diode 30 is connected to the cathode of the diode 14,and the cathode of the diode 30 is connected to one electrode of thecapacitor 31. The other electrode of the capacitor 31 is connected tothe cold side of the auxiliary winding 2 c of the transformer 2. Theresistor 32 is connected between both terminals of the capacitor 31.

Flyback converters may not generate a voltage for driving the gate ofthe NMOS 16 from the cold side of the secondary winding 2 c when theoutput voltage Vo is low, for example, during the starting period.Consequently, even if a current which flows through the reactor 5becomes zero and the transistor 18 turns off, a predetermined voltage isnot generated at the gate of the NMOS 16, and this results in theindeterminate gate voltage of the NMOS 7. In this manner, in a casewhere the NMOS 3 as the main switching element turns on, and the voltageat the source of the NMOS 7 becomes low with respect to the voltage atthe drain of the NMOS 7 with the voltage at the gate of the NMOS 7 beingindeterminate, an input capacitance is charged through a feedbackcapacitance of the NMOS 7, and a voltage is generated at the gate of theNMOS 7. Because of this gate voltage, there is fear that the NMOS 7turns on and a penetration current flows.

In contrast, in the switching power supply device of the embodiment, ina case where the NMOS 16 is in an off state and the NMOS 3 turns on, thevoltage at the source of the NMOS 7 becomes low with respect to thedrain thereof, and the electric potential of the cold side of theauxiliary winding 2 c becomes further low with respect to the electricpotential of the source of the NMOS 7. Accordingly, the feedbackcapacitance of the NMOS 7 is charged through the diode 14, the diode 30,and the capacitor 31. As the electric potential of the cathode of thediode 14 becomes low with respect to the source of the NMOS 7 by theforward voltage of the parasitic diode of the NMOS 16, the gate voltageof the NMOS 7 becomes almost 0V, and does not turn on. Moreover, nonegative overload voltage is applied.

(Seventh Embodiment)

FIG. 8 is a circuit diagram illustrating a switching power supply deviceaccording to a seventh embodiment of the invention, and componentscommon to those of the sixth embodiment in FIG. 7 will be denoted by thesame reference numerals.

The switching power supply device includes a diode 33 instead of theresistor 32 of the switching power supply device of the sixthembodiment. The anode of the diode 33 is connected to the node betweenthe cathode of the diode 30 and the capacitor 31, and the cathode of thediode 33 is connected to the collector of the transistor 13.

In the sixth embodiment, the charge stored in the capacitor 31 in an onperiod of the NMOS 3 is discharged by the resistor 32, but in theswitching power supply device of the embodiment, the charge stored inthe capacitor 31 is supplied to the gate of the NMOS 7 through thecollector of the transistor 13. That is, the charge stored in thecapacitor 31 is used for driving the NMOS 7, and this results in makingefficient use of the charge.

(Eighth Embodiment)

FIG. 9 is a circuit diagram illustrating a switching power supply deviceaccording to an eighth embodiment of the invention, and the componentscommon to those of the sixth embodiment in FIG. 7 will be denoted by thesame reference numbers.

The switching power supply device eliminates the transistor 13 anddiodes 10, 14 in the switching power supply device of the sixthembodiment, and includes diodes 34, 35.

The one electrode of the capacitor 31 is connected to the gate of theNMOS 7 via the resistor 15, and the other electrode of the capacitor 31is directly connected to the cold side of the auxiliary winding 2 c. Theanode of the diode 34 is connected to the one electrode of the capacitor31, while the cathode of the diode 34 is connected to the node betweenthe resistor 24 and the drain of the NMOS 16. The anode of the diode 35is connected to the source of the NMOS 7, while the cathode of the diode35 is connected to the gate of the NMOS 7 through the resistor 15.

In this switching power supply device, the synchronous rectificationNMOS 7 is driven by the capacitor 31. The capacitor 31 is charged by thevoltage induced in the secondary winding 2 c through the diode 35 whenthe NMOS 3 as the main switching element is on. When the NMOS 3 as themain switching element turns off and a voltage at the auxiliary winding2 c is inverted, the NMOS 7 is turned on by the charge stored in thecapacitor 31 and the voltage induced in the auxiliary winding 2 c. Thecapacitor 31 is enough if it can drive the gate of the NMOS 7, thus thecapacitance of the capacitor 31 may be a small value.

As the current IL which flows through the reactor 5 decreases and theNMOS 16 turns on, the NMOS 16 turns on. Therefore, the electricalcharges are released from the gate of the NMOS 7 through the diode 34 sothat the NMOS 7 turns off. Thus, the capacitor 31 becomes a state thatthe one electrode is connected to the hot side of the auxiliary winding2 c, and inversely charged. The charge of the capacitor 31 is keptundergone until the charge voltage of the capacitor 31 becomes equal tothe voltage which is generated by the auxiliary winding 2 c. After that,no current flows through the auxiliary winding 2 c. Thus, the loss ofthe power is small. Further, the capacitance of the capacitor 31 may berelatively small, thus the valued of the current flowing through theauxiliary winding 2 c is small.

The diode 34 prevents the current IL which flows through the reactor 5from flowing back into the one electrode of the capacitor 31.

In the above-described switching power supply apparatus, the transistor13 can be eliminated, and the NMOS 7 can be driven by the capacitor 31which is cheaper than the transistor 13. It is possible to reduce thenumber of elements of the diodes and the resistors, thereby reducing thecost of the switching power supply device.

(Ninth Embodiment)

FIG. 10 is a circuit diagram illustrating a switching power supplydevice according to a ninth embodiment of the invention, and componentscommon to those of the seventh embodiment in FIG. 8 will be denoted bythe same reference numerals.

The switching power supply device comprises a Zener diode 36.

The cathode of the Zener diode 36 is connected to the cathode of thediode 10, while the anode of the Zener diode 36 is connected to the oneend of the resistor 12, and the other end of the resistor 12 isconnected to the base of the transistor 13.

In the switching power supply device, in a case where the output voltageVo is low during starting or the like and the voltage which is generatedby the auxiliary winding 2 c is low, the Zener diode 36 prevents flow ofthe base current into the transistor 13. This makes it possible tosuppress an unstable operation which is likely to occur during thestarting or the like.

(Tenth Embodiment)

FIG. 11 is a circuit diagram illustrating a switching power supplydevice according to a tenth embodiment of the invention, and componentscommon to those of the fourth embodiment in FIG. 5 will be denoted bythe same reference numerals.

The switching power supply device comprises the DC power source 1, thetransformer 2, the NMOS 3 which is the main switching element, and thecontroller which controls on/off operations of the NMOS 3. The DC powersource 1 and the NMOS 3 are connected to the transformer 2 in similarways to those of the first to ninth embodiments.

The cold side of the secondary winding 2 b of the transformer 2 isconnected to the one end of the resistor 11 and the one electrode of thecapacitor 6. The other electrode of the capacitor 6 is connected to theground GND.

The hot side of the secondary winding 2 b is connected to one end of aresistor 37, the cold side of the auxiliary winding 2 c, and the drainof the synchronous rectification NMOS 7. The other end of the resistor37 is connected to one end of a resistor 38, and the other end of theresistor 38 is connected to the source of the NMOS 7. The source of theNMOS 7 is connected to the ground GND.

The hot side of the auxiliary winding 2 c is connected to the one end ofthe reactor 5. The other end of the reactor 5 is connected to the anodesof the diodes 8, 20, and 23. The cathode of the diode 8 is connected tothe ground GND via the resistor 9, and is connected to the base of thetransistor 18.

The cathode of the diode 20 is connected to the collector of thetransistor 18, and the emitter of the transistor 18 is connected to theground GND. The anode of the diode 17 is connected to the other end ofthe resistor 11, while the cathode of the diode 17 is connected to thecollector of the transistor 18.

The cathode of the diode 17 is further connected to the gate of the NMOS16. The cathode of the diode 23 is connected to the one end of theresistor 24, and the other end of the resistor 24 is connected to thedrain of the NMOS 16. The source of the NMOS 16 is connected to theground GND.

The node between the resistor 37 and the resistor 38 is connected to oneinput terminal of a two-input NOR circuit 39. The other input terminalof the NOR circuit 39 is connected to the collector of the transistor18. The output terminal of the NOR circuit 39 is connected to the gateof the NMOS 7 via the resistor 15.

In the switching power supply device connected as described above, thereactor 5 stores and releases energy in accordance with the voltagewhich is generated by the auxiliary winding 2 c. The resistor 9 detectsthe current IL which flows through the reactor 5, and as similar to theswitching power supply devices of the first to ninth embodiments, thetransistor 18 turns on based on the current which flows through thereactor 5.

The on action of the transistor 18 causes a low-level signal to be inputinto the other input terminal of the NOR circuit 39. The off action ofthe transistor 18 causes a high-level signal to be input into the otherinput terminal of the NOR circuit 39.

The resistances 37 and 38 in series are connected between the source anddrain of the NMOS 7. Therefore, when the NMOS (main switch) 3 is on, thevoltage across the secondary winding 2 b and the output voltage Vo isapplied to the resisters 37 and 38. When the NMOS 3 is off, theparasitic diode of the NMOS 7 is forward-biased. Therefore, a lowvoltage is applied to the resistors 37 and 38. Therefore, the connectionnode between the resistors 37 and 38, i.e., one input terminal of theNOR circuit 39 is at a high voltage when the NMOS 3 is on, and at a lowvoltage when the NOMS 3 is off.

Therefore, the NOR circuit 39 outputs a high-voltage level signal when acurrent grater than a predetermined current level flows through thereactor 5 to turns on the transistor 18 and the NMOS 3 as the mainswitch is off. The high-voltage level output signal from the NOR circuit39 drives or turned on the NMOS 7.

In the switching power supply devices of the first to ninth embodiments,as the gate of the NMOS 7 is driven by the voltage which is generated bythe auxiliary winding 2 c, it is difficult to set the voltage which isgenerated by the auxiliary winding 2 c at a low level too much, but inthis embodiment, as the auxiliary winding 2 c is used only for storingand releasing the energy of the reactor 5, it is possible to reduce thenumber of turns ns, and this makes it possible to replace the reactor 5by a small and inexpensive one.

(Eleventh Embodiment)

FIG. 12 is a circuit diagram illustrating a switching power supplydevice according to an eleventh embodiment of the invention, andcomponents common to those of the tenth embodiment in FIG. 11 will bedenoted by the same reference numerals.

This switching power supply device is the switching power supply deviceof the tenth embodiment which eliminates the diodes 17, 20, the resistor11, the transistor 18, the NMOS 16, and the NOR circuit 39, but includesa comparator 40, a reference voltage generator 41, a diode 42, aninverter 43, and an AND circuit 44.

The diode 8 with its anode connected to the other end of the reactor 5is connected to the one end of the resistor 9 as same as the tenthembodiment, and is connected to one input terminal of the comparator 40,and the anode of the diode 42. The cathode of the diode 42 is connectedto the other end of the resistor 9 and the ground GND. A referencevoltage which is generated by the reference voltage generator 41 isinput into the other input terminal of the comparator 40. The referencevoltage is lower than the forward voltage of the diode 42.

The cathode of the diode 23 with the anode connected to the otherelectrode of the reactor 5 is connected to the one end of the resistor24, and the other end of the resistor 24 is connected to the outputterminal of the comparator 40.

As same as the tenth embodiment, the series circuit of the resistors 37,38 is connected between the drain of the NMOS 7 and the source thereof.The input terminal of the inverter 43 is connected to the node betweenthe resistor 37 and the resistor 38, while the output terminal of theinverter 43 is connected to one input terminal of the AND circuit 43.The other input terminal of the AND circuit 43 is connected to theoutput terminal of the comparator 40. The output terminal of the ANDcircuit 43 is connected to the drain of the NMOS 7 via the resistor 15.

In the switching power supply device, the comparator 40 compares thevoltage drop at the resistor 9 with the reference voltage generated bythe reference voltage generator 41 and output a high level signal to oneinput terminal of the AND circuit 44 when the voltage drop at theresistor 9 is greater than the reference voltage, that is, a currentgreater than a predetermined level flows through the reactor 5. Theresistors 37 and 38 are connected between the drain and source of theNMOS 7 in series. Therefore, the connection node between the resistors37 and 38 is at a high level when the NMOS 3 is on, and is at a lowlevel when the NMOS 3 is off. The connection node of resistors 37 and 38is connected to another input terminal of the AND circuit 44 through aninverter circuit 43. Therefore, the AND circuit 44 output a high levelsignal when the NMOS 3 is on, and the current greater than thepredetermined current level is flowing through the reactor 5, to turnson the NMOS 7. The diode 42 clamps the voltage drop at the register 9 atthe forward voltage of it to protect the comparator 40. In thisembodiment, the detection of the current flowing through the reactor 5is performed by comparing the voltage drop at resistor 9 and thereference voltage. The reference voltage has a small change due to atemperature change in comparison with the base-emitter voltage of atransistor. Accordingly, the comparison result of the comparator 40becomes stable. Therefore, changing in the on/off timings of the NMOS 7in accordance with a temperature change can be suppressed.

Various embodiments and changes may be made thereunto without departingfrom the broad spirit and scope of the invention. The above-describedembodiments are intended to illustrate the present invention, not tolimit the scope of the present invention. The scope of the presentinvention is shown by the attached claims rather than the embodiments.Various modifications made within the meaning of an equivalent of theclaims of the invention and within the claims are to be regarded to bein the scope of the present invention.

This application is based on Japanese Patent Application No. 2004-336008filed on Nov. 19, 2004 and including specification, claims, drawings andsummary. The disclosure of the above Japanese Patent Application isincorporated herein by reference in its entirety.

1. A switching power supply device comprising: a transformer with aprimary and a secondary winding; a main switching element which switchcsa current flowing through said primary winding; a controller whichcontrols an operation of said main switching element; a smoothingcircuit; a rectifying switching element which connects and disconnectsbetween said secondary winding and said smoothing circuit; and arectifying-element drive circuit which drives said rectifying switchingelement, and includes a reactor of which one end is connected to one endof said secondary winding stores energy during an on period of said mainswitching element, and releases said stored energy during an off periodof said main switching element, a current detection circuit which isconnected between an other end of said reactor and an other end of saidsecondary winding, detects the current value of the current flowingthrough the reactor, and which generates a signal indicating the currentvalue, and a drive circuit which receives said signal indicating saidcurrent value, and turns on said rectifying switching element in saidoff period of said main switching element when said current flowingthrough said reactor is greater than or equal to a predetermined value,and turns off said rectifying switching element when said currentflowing through said reactor is less than said predetermined value. 2.The switching power supply device according to claim 1, wherein saidcurrent detection circuit includes a current detection resistor havingone end connected to said other end of said reactor and an other endconnected to said other end of said secondary winding, and said drivecircuit includes a transistor with a control electrode (base), and afirst and a second conduction electrode (emitter, collector) whichchange conduction states based on a signal supplied to said controlelectrode, said control electrode (base) being connected to said otherend of reactor, said first conduction electrode (emitter) beingconnected to said other end of secondary winding, and said drive circuitreceives a voltage drop at said current detection resistor as saidsignal indicating said current value, and turns off said rectifyingswitching element, when the voltage drop at the current detectionresistor due to said current flowing through said reactor becomes lowerthan a threshold of said transistor.
 3. The switching power supplydevice according to claim 2, wherein said transistor is a bipolartransistor whose base, emitter, and collector respectively correspond tosaid control electrode, said first conduction electrode, and said secondconduction electrode.
 4. The switching power supply device according toclaim 2, wherein said transistor is a MOS transistor whose gate, source,and drain respectively correspond to said control electrode, said firstconduction electrode, and said second conduction electrode.
 5. Theswitching power supply device according to claim 2, wherein said drivecircuit includes an off-control switch which has a current path with oneend (source) connected to said secondary winding, and an other end(drain) connected to a control terminal of said rectifying switchingelement, and a control terminal (gate) connected to an auxiliary windingconnected to said secondary winding in series, and said secondconduction electrode of said transistor, turns on, reducing a voltage atsaid control terminal of said rectifying switching element, when saidtransistor is turned off.
 6. The switching power supply device accordingto claim 2, wherein said drive circuit includes a current bypass diodewhose anode and cathode respectively connected to said reactor, and saidsecond conduction electrode of said transistor.
 7. The switching powersupply device according to claim 5, wherein said drive circuit includesa hysteresis circuit which includes a resistor and a diode, is connectedbetween said reactor and said other end of said current path of saidoff-control switch, and permits said current flowing through saidreactor to partly flow through said off-control switch during an onperiod of said off-control switch.
 8. The switching power supply deviceaccording to claim 5, wherein said drive circuit includes a bias circuitwhich ensures a current flow to said current detection circuit from saidauxiliary winding or said control terminal of said rectifying switchingelement via a resistor.
 9. The switching power supply device accordingto claim 5, wherein said rectifying-element drive circuit includes acapacitor which is connected between said control terminal of saidrectifying switching element and said auxiliary winding, and has afunction of reducing a voltage at said control terminal of saidrectifying switching element.
 10. The switching power supply deviceaccording to claim 5, wherein said rectifying-element drive circuitincludes a capacitor which is connected between said control terminal ofsaid rectifying switching element and said auxiliary winding, and has afunction of reducing a voltage at said control terminal of saidrectifying switching element, and a function of driving said rectifyingswitching element.
 11. The switching power supply device according toclaim 5, wherein said drive circuit includes a drive transistor whoseemitter, base, and collector are respectively connected to said controlterminal of said rectifying switching element, an other end of a mainterminal of said off-control switch, and said auxiliary winding, and aresistor and a Zener diode are connected between said base and saidcollector of said drive transistor.
 12. The switching power supplydevice according to claim 1, wherein said drive circuit includes acurrent detection circuit which detects a current value of the currentflowing through the reactor and a gate circuit which turns on and offsaid rectifying switching element based on the detection result of thecurrent detection circuit when said main switching element is off. 13.The switching power supply device according to claim 1, wherein saidcurrent detection circuit includes a current detection resistor havingone end connected to said reactor and an other end connected to saidsecondary winding, and a comparator which compares a voltage generatedby said current detection resistor with a predetermined voltage, andsaid drive circuit turns on and off said rectifying switching elementbased on an output signal of said comparator when said main switchingelement is off.
 14. A switching method comprising the steps of:intermittently supplying a current to a primary winding of atransformer; and turning on a rectifying switching element to supply anoutput of a secondary winding of said transformer to a smoothing circuitvia a rectifying switching element by in a period in which no currentflows through said first winding and when a current value of a currentflowing through a reactor connected, with a current detection circuit,between both end of said secondary winding, connected to said secondarywinding of said transformer in parallel, is greater than or equal to apredetermined value, based on said current flowing through said reactor,and turning off said rectifying switching element when said currentvalue of said current flowing through said reactor is less than saidpredetermined value.
 15. A switching power supply device comprising: atransformer with a primary and a secondary winding; a main switchingelement which switches a current flowing through said primary winding; acontroller which controls an operation of said main switching element; asmoothing circuit; a rectifying switching element which connects anddisconnects between said secondary winding and said smoothing circuit;and a rectifying-element drive circuit which drives said rectifyingswitching element, and includes a reactor of which one end is connectedto one end of said secondary winding stores energy during an on periodof said main switching element, and releases said stored energy duringan off period of said main switching element, and a drive circuit whichreceives said signal indicating said current value, and turns on saidrectifying switching element in said off period of said main switchingelement when said current flowing through said reactor is greater thanor equal to a predetermined value, and turns off said rectifyingswitching element when said current flowing through said reactor is lessthan said predetermined value, said driving circuit includes a currentdetection circuit which detects the current value of the current flowingthrough the reactor, and said current detection circuit includes acurrent detection resistor with one end connected to said reactor and another end connected to said secondary winding, and a transistor with acontrol electrode (base), and a first and a second conduction electrode(emitter, collector) which change conduction states based on a signalsupplied to said control electrode, said control electrode (base) beingconnected to said reactor, said first conduction electrode (emitter)being connected to said secondary winding, and said drive circuit turnsoff said rectifying switching element, based on a voltage at said secondconduction electrode of said transistor, when a voltage drop at saidcurrent detection resistor due to said current flowing through saidreactor becomes lower than a threshold of said transistor, and whereinsaid drive circuit includes an off-control switch which has a currentpath with one end (source) connected to said secondary winding, and another end (drain) connected to a control terminal of said rectifyingswitching element, and a control terminal (gate) connected to anauxiliary winding, connected to said secondary winding in series, andsaid second conduction electrode (collector) of said transistor, turnson, reducing a voltage at said control terminal of said rectifyingswitching element, when said transistor is turned off.
 16. The switchingpower supply device according to claim 15, wherein said drive circuitincludes a hysteresis circuit which includes a resistor and a diode, isconnected between said reactor and said other end of said current pathof said off-control switch, and permits said current flowing throughsaid reactor to partly flow through said off-control switch during an onperiod of said off-control switch.
 17. The switching power supply deviceaccording to claim 15, wherein said drive circuit includes a biascircuit which ensures a current flow to said current detection circuitfrom said auxiliary winding or said control terminal of said rectifyingswitching element via a resistor.
 18. The switching power supply deviceaccording to claim 15, wherein said rectifying-element drive circuitincludes a capacitor which is connected between said control terminal ofsaid rectifying switching element and said auxiliary winding, and has afunction of reducing a voltage at said control terminal of saidrectifying switching element.
 19. The switching power supply deviceaccording to claim 5, wherein said rectifying-element drive circuitincludes a capacitor which is connected between said control terminal ofsaid rectifying switching element and said auxiliary winding, and has afunction of reducing a voltage at said control terminal of saidrectifying switching element, and a function of driving said rectifyingswitching element.
 20. The switching power supply device according toclaim 15, wherein said drive circuit includes a drive transistor whoseemitter, base, and collector are respectively connected to said controlterminal of said rectifying switching element, an other end of a mainterminal of said off-control switch, and said auxiliary winding, and aresistor and a Zener diode are connected between said base and saidcollector of said drive transistor.
 21. The switching power supplydevice according to claim 15, wherein said transistor is a bipolartransistor whose base, emitter, and collector respectively correspond tosaid control electrode, said first conduction electrode, and said secondconduction electrode.
 22. The switching power supply device according toclaim 15, wherein said transistor is a MOS transistor whose gate,source, and drain respectively correspond to said control electrode,said first conduction electrode, and said second conduction electrode.23. The switching power supply device according to claim 15, whereinsaid drive circuit includes a current detection circuit which detects acurrent value of the current flowing through the reactor and a gatecircuit which turns on and off said rectifying switching element basedon the detection result of the current detection circuit when said mainswitching element is off.
 24. The switching power supply deviceaccording to claim 15, wherein said current detection circuit includes acurrent detection resistor having one end connected to said reactor andan other end connected to said secondary winding, and a comparator whichcompares a voltage generated by said current detection resistor with apredetermined voltage, and said drive circuit turns on and off saidrectifying switching element based on an output signal of saidcomparator when said main switching element is off.